Valuable materials are extracted from ores by beneficiation processes. One beneficiation process comprises washing wherein the valuable materials are separated from the undesired materials on the basis of specific gravity difference by first forming a slurry of the crushed ore. Essential to any process of separation based upon specific gravity difference is the physical separation of the valuable materials and the undesired materials in the slurry. This application is directed to washing processes improved by better separation in the slurry brought about by the efficient application of ultrasound energy to the slurry. Coal washing is one example of beneficiation by a separation based upon specific gravity difference between the valuable and the undesired materials.
Coal, as a fuel, is an abundant resource of energy comprised mostly of carbon and small percentages of hydrogen, sulfur and ash. When coal is burned to produce energy, the presence of sulfur and ash is generally undesirable. The ash enters the atmosphere as small particles (particulates) and the sulfur as noxious sulfur oxide gases. In the Appalachian coal fields, conventional coal mining operations have traditionally refined useful coal by washing from it less valuable sand and clay, although fine particles of coal are often agglomerated with the residues produced by coal washing processes. Because fine coal remains a part of these residues, so-called "gob piles" and coal slurry ponds of residues have accumulated throughout the coal fields adjacent to coal washing yards. These stock piles are rich in fine particles of useful coal. In the past, it has proved to be economically impractical to recover this coal.
Sulfur is present in coal in three principal forms: pyritic sulfur (a combination of iron and sulfur), sulfate sulfur and organic sulfur, that is, chemically combined sulfur within the coal structure. Pyritic sulfur can, to a large extent, be Washed out of coal by conventional coal washing methods. These methods are not, however, suitably efficient on a large scale and, at best, only a small portion of the mined coal can be sufficiently upgraded by washing alone. Sulfate sulfur can be separated from coal by dissolving it in water. For example, it may be boiled out of the coal matrix by elevated temperature processes which have already been developed. At the present time, there appears to be no commercial process for removing substantial amounts of organic sulfur from coal.
Numerous processes have been proposed for upgrading coal to remove various forms of sulfur. The following have been considered: (1) oxidation of sulfur in the coal in an aqueous medium to form soluble sulfates; (2) reduction of the sulfur to elemental sulfur in which form it can be vaporized or removed by organic solvents; (3) reaction with hydrogen to form gaseous hydrogen sulfide; (4) vapor deposition selectively on the pyritic form of sulfur followed by magnetic separation of the pyrites; (5) oxidation of the sulfur with nitric oxide vapors to form gaseous sulfur oxides; (6) leaching with a sodium and calcium oxide lixiviant; and (7) leaching with aqueous ferric sulfate.
Ultrasound treatment of coal slurries has been demonstrated to be useful in coal washing processes as disclosed, for example, in U.S. Pat. Nos. 4,391,608 and 4,919,807. Typically, the slurry is passed through a substantially horizontal trough to which ultrasound transmitters are attached.
The applicant's process disclosed herein is directed to the removal of one or more of the three basic forms of sulfur from coal and coal-like materials. At the same time, the process reduces the amount of ash within the coal or coal-like material. The recovery of useful coal is improved and the ability of the coal product to shed water is increased. The process involves the use of atmospheric pressures and low temperatures (temperature near room temperature) and may be practiced with rugged processing equipment. Further, the process improves the characteristics of the residue thereby enhancing their disposition and reducing problems with their disposal and allowing potential beneficial uses of them.
It should be understood that the processes and apparatus disclosed herein have application not only to coal washing but also to the beneficiation of mineral ores, for example, gold ore and tailings from gold ore beneficiation, wherein the valuable materials and gangue (unwanted materials) can be separated on the basis of specific gravity difference. Indeed, in some ways these processes and apparatus have greater application to separation of heavier and more abrasive materials than raw coal. An additional advantage of this invention is the minimization of the abrasive action of the slurry on the apparatus for applying ultrasound to the slurry.
The inventor does not wish to be limited by any particular theory of operation but it is understood that sound waves in liquids comprise expansion cycles and compression cycles. During the expansion cycle, the molecules of the liquid are separated creating a gap or "cavity" in the liquid. The cavity only exists until the next compression cycle at which time the cavity rapidly implodes. This implosion creates a microscopic jet directed at or along the surface of solids within the liquid and also rapidly heats the liquid surrounding the cavity (which is almost instantaneously cooled by the large mass of liquid). The combination of extremely hot liquid and microscopic jets work upon the surfaces of the solids.
In the past, ultrasound energy having frequencies in the range of about 10 to 50 kHz have been suggested for treatment of coal slurries. This was based in part upon the expected attenuation of sound energy as it progresses into the slurry. The higher the frequency, the more rapid the attenuation. According to the invention disclosed herein, much higher frequencies are contemplated. The doubling of frequency doubles the number of locations within the slurry about which cavitation and implosion take place, thus increasing the action of the ultrasound on smaller and smaller particles because the implosion locations are closer to one another.